Scientists believe the world's amphibians are facing an unprecedented onslaught of environmental threats.

They say as many as 122 species may have become extinct since 1980 and a third of known amphibians face oblivion.

Naturalists describe the creatures as sensitive indicators of the health of the wider environment.

The Global Amphibian Assessment, the work of more than 500 scientists, is published in Science Express, the

online version of the journal Science.

Their catastrophic decline serves as a warning that we are in a period of significant environmental degradation

Russell Mittermeier, Conservation International

It was compiled by an international team from Conservation International, IUCN-The World Conservation Union, and NatureServe, and is the first comprehensive assessment of the threat.

Skin alert

There are 5,743 known amphibians, a category which includes frogs, toads, salamanders and caecilians (legless amphibians).

Of these, 1,856 - almost a third - are now judged to be at risk of extinction. At least nine species have slipped over the edge to oblivion since 1980, when the assessment says the most dramatic declines began.

The harlequin toad from Costa Rica and Panama has been hit hard by chytridiomycosis

Another 113 amphibians which have not been seen in the wild for some years are thought to be possibly extinct.

The scientists say 43% of all amphibians are declining, 27% are stable, under 1% are increasing, and the status of the rest is unknown.

They describe amphibians as "the canaries in the coal mine", as their highly permeable skins are very sensitive to environmental changes, including in water and air quality.

Russell Mittermeier, president of Conservation International, said: "Amphibians are one of Nature's best indicators of overall environmental health.

"Their catastrophic decline serves as a warning that we are in a period of significant environmental degradation."

'Mass extinction'

Achim Steiner, director-general of IUCN, said: "After birds and mammals, amphibians are the third group of species to be completely evaluated on a global scale.

WHAT ARE AMPHIBIANS?

Group includes frogs, toads, salamanders and caecilians

First true amphibians evolved about 250m years ago

Adapted to many different aquatic and terrestrial habitats

Present today on every continent except Antarctica

Undergo metamorphosis, from larvae to adults

"This study significantly expands our current knowledge and provides a baseline from which we can monitor our impact on the environment over time.

"The fact that one third of amphibians are in a precipitous decline tells us that we are rapidly moving towards a potentially epidemic number of extinctions."

The IUCN Red List of Threatened Species says 12% of all birds and 23% of mammals risk becoming extinct, well below the 32% of threatened amphibians.

Water critical

Colombia, Mexico, Ecuador, Brazil and China each have large numbers of species at risk, and 92% of Haiti's amphibian species are in danger.

A highly infectious fungal disease, chytridiomycosis, is attacking many amphibians in the Americas, the Caribbean and Australia.

The crocodile newt is over-harvested for use in traditional medicines

In some areas outbreaks are thought to be linked to climate-induced drought.

Elsewhere, though, the researchers say, habitat destruction, air and water pollution and consumer demand are among the chief culprits for the amphibians' plight.

Simon Stuart, who led the researchers, said: "Since most amphibians depend on fresh water and feel the effects of pollution before many other forms of life, including humans, their rapid decline tells us that one of Earth's most critical life support systems is breaking down."